In previous studies we have shown that the synthesis of lactose by incubating lactating guinea pig mammary gland slices can be completely inhibited by the phosphodiesterase (PDE) inhibitors theophylline and papaverine and only partially inhibited by normal and dibutyryl cyclic 3',5' adenosine monophosphate (cAMP) and by epinephrine and norepinephrine. These observations suggest a regulatory role for cAMP as a physiological inhibitor of lactose production. Regulation of the latter, being the major osmole in milk, probably also regulates fluid transport and, therefore, milk flow. During the past year we have extended these studies to show that 1-methyl 3-isobutyl xanthine (MIX), a more specific PDE inhibitor, has the same inhibitory effect on lactose production although we were not able to demonstrate additive inhibitory effects between MIX and cAMP. In addition, compounds related to cAMP including 5' AMP, ATP, and cyclic 3',5' guanosine monophosphate did not have the inhibitory effect of cAMP at the same concentration (10-4M). We have also found that the microtubule-disrupting drug colchicine at 10-5 to 10-4M is also inhibitory; however, this drug appears to act primarily on release of lactose into the media from alveolar cells and secondarily on its synthesis. During the second year of the grant period we will extend these studies by examining endogenous levels of cyclic nucleotides and related enzymes when rate of lactose production is manipulated in vitro with exogenous nucleotides, PDE inhibitors, hormones, or prostaglandins. BIBLIOGRAPHIC REFERENCES: Klein, D. M., Loizzi, R.F. and Guerin, M.A. Enhancement of mammary tumor growth and cellular differentiation by dibutyryl cyclic AMP. J. Cell Biol. 67: 216a, 1975. Loizzi, R. F., de Pont, J.J. and Bonting, S.L. Theophylline and cyclic AMP inhibition of lactose production in lactating guinea pig mammary gland slices. Adv. in Cyclic Nuceotide Res. 5: 804, 1975.